In recent years, there are increasing needs for electronic paper, which is an electronic medium to replace paper, and development activities for electronic paper are flourishing. As a means for realizing display systems for such electronic paper, self-luminescent display techniques such as liquid crystal displays and organic EL displays have been developed, and some of them have been commercialized. Meanwhile, reflective display techniques that are low power consumptive and excellent in visibility are seen as promising display techniques for the next-generation electronic paper.
A widely known reflective display technique is an electrophoretic system configured to inverse the positions of electrically charged white and black particles with an electric field and switch white and black display positions, and this system is employed in electronic book readers, etc. However, conventional techniques are limited to monochrome display, and full-color reflective display techniques are highly demanded. In this connection, as such a full-color reflective display technique, there is known a full-color display system that realizes color display with RGB (W) color filters overlaid over the monochrome electrophoretic system. However, there has been a problem that color vividness and brightness are poor because a space is divided by three or four colors.
Hence, as a promising technique for realizing a reflective display element without color filters, an electrochromic display system that utilizes an electrochromism phenomenon is paid attention. An electrochromic display system can expect a high color reproducibility and a display memory property.
An electrochromism phenomenon is a phenomenon that a specific substance causes a reversible oxidation-reduction reaction upon application of a voltage and changes to various colors. An electrochromic display device is a display device utilizing color development and color fading (hereinafter, referred to as color development and fading) of an electrochromic compound that causes such an electrochromism phenomenon.
Research and development for such electrochromic display devices are conducted from a wide variety of aspects from material development to device designing, because they are an important candidate for a display device technique for electronic paper for the reasons that they can produce various colors by engineering of the molecules of an organic electrochromic compound, that they are reflective display devices, that they have a display memory property, that they can be driven at a low voltage, etc.
Electrochromic display devices are expected as multi-color display devices because they can produce various colors depending on the structure of the electrochromic compound. Electrochromic display devices are one of electrochemical elements that apply a current typically across a pair of electrodes facing each other and utilize a color development reaction due to an oxidation-reduction reaction of an electrochromic compound provided between the electrodes. In order to realize a vivid full-color display, they need to have a structure in which three primary colors, namely yellow, cyan, and magenta are overlaid based on a subtractive color mixing method. As an example of this, a full-color display technique (multi-color display technique) based on overlaying of three element layers for yellow, cyan, and magenta has been reported (see PTL 1).
As the multi-color display technique, there are proposed configurations in which a plurality of display electrode layers and electrochromic color development layers are stacked over one display substrate (see PTLs 1 to 4). Further, there are proposed electrochromic display devices that use active-matrix TFTs as counter electrodes facing display electrodes (see PTLs 1 and 2). These proposals are characterized in that the plurality of display electrodes need not be finely patterned, and that a full-color display image can be obtained at a high aperture ratio with three display electrodes switched by one active-matrix TFT panel.
However, in these proposals, the plurality of display electrodes are common to the respective pixels, which leads to crosstalk between the pixels (color blurring and resolution degradation) resulting in blurring of a display image, and to crosstalk between the plurality of display electrodes (color mixing between the plurality of layers) resulting in a problem in a display image retention property. That is, the problem is intra-layer and inter-layer crosstalk.
Further, as a method for suppressing occurrence of crosstalk between pixels of a passive matrix driven electrochromic display device, there are proposed configurations in which a nickel oxide layer is provided between a display electrode and an electrochromic layer (see PTLs 3 and 4). There is also proposed a technique for shifting a response voltage from an electrochromic layer, by means of a diode structure formed by stacking a layer of titanium oxide known to be an n-type semiconductor and a layer of nickel oxide known to be a p-type semiconductor (see PTL 4).
However, a nickel oxide layer from which semiconductor properties can be obtained has a black color, which is a factor of reducing display qualities such as color vividness and brightness. Hence, improvement in the optical properties of the nickel oxide layer itself is demanded.